1. Field of the Invention
The present invention relates to an exhaust purifier for an internal combustion engine, and particularly, to one that heats a catalyst with an electric heater in consideration of a deterioration with time and manufacturing variation in the heater.
2. Description of the Related Art
An exhaust system of an internal combustion engine employs a catalyst in an exhaust passage to remove toxic substances such as HC, CO, and NOx from the exhaust gas. The catalyst purifies exhaust gas only after it is heated above an activation temperature. To heat the catalyst, hot exhaust gas from the engine is usually used. If the engine is started from a cold state, it will take time to heat the catalyst to the activation temperature because the temperature of exhaust gas is low at first. Until the catalyst is activated, the exhaust gas is insufficiently purified.
To solve this problem, an electrically heated catalyst (EHC) has been proposed. The EHC has a catalyst and a metal carrier to which a current is applied at the start of the engine to quickly heat the catalyst up to an activation temperature. An example of the EHC is disclosed in Japanese Unexamined Patent Publication No. 5-179939. The EHC, however, consumes large amounts of power, dropping the voltage of a battery during an idling period.
To prevent such voltage drop, Japanese Unexamined Patent Publication No. 6-101459 checks an idling state and a heater operation. If the heater is operating while the engine is idling, the disclosure increases the quantity of intake air by controlling an electronic throttle or an idling speed controller (ISC) disposed in an intake system of the engine. As the quantity of intake air increases, the quantity of injected fuel increases to increase the torque and idling speed of the engine. This increases the power generated by a generator to compensate for the voltage drop of the battery.
This disclosure determines a period for operating the heater according to the temperature of cooling water of the engine without considering a deterioration with time of the heater resistance and manufacturing variation in the heater. Accordingly, the heater operating period may be too long or too short for the catalyst. As a result, the heater and the heat of exhaust gas will excessively or insufficiently heat the catalyst.
An object of the present invention is to provide an exhaust purifier for an internal combustion engine, capable of properly heating a catalyst in consideration of a deterioration with time and manufacturing variation in an electric heater that heats the catalyst.